Tephrostratigraphy in the Ulleung Basin, East Sea: Late Pleistocene to Holocene

Abstract

Three sediment cores, obtained from the Ulleung Basin, reveal five tephra layers that are easily distinguished from the other core intervals by coarse grain size, density grading, and sediment color. The tephra layers are well correlated among the cores and show a systematic lateral decrease in thickness and grain size away from the source of the explosive volcanic eruption. Three marker tephra layers were derived from Ulleung Island and consist predominantly of white vesiculate pumices of phonolitic or trachytic alkali magma. The disorganized Ulleung-I tephra layer was due to the last explosive eruption in Ulleung Island. It consists of only white vesiculate pumices, disturbed by strong bioturbation. The Ulleung-II tephra (ca. 9300 yr B.P.), the most explosive one among the Ulleung Island-derived tephras, shows two units: a lower density-graded unit and an upper pumice-scattered unit. The Ulleung-II tephra layer shows little bioturbation and is characterized by pumice-scattered texture. An unnamed tephra layer shows a distinct density grading by dark fine-grained crystals and white vesiculate pumices. This marker tephra originated from a phonolitic alkali magma of unknown source. The Aira-Tanzawa ash (ca. 22000 yr B.P.), derived from the Aira Caldera in southern Japan, consists commonly of bubblewalled glass shards from rhyolitic subalkali magma. During the Holocene period, sediments were accumulated at a rate of 17.5–24.7 cm/kyr, based on the Ulleung-II tephra, which increased toward the center of the basin. During the last glacial maximum (LGM) period which is represented by the interval between the Ulleung-II tephra layer and the Aira-Tanzawa ash, a very thick turbidite mud was deposited in the unchannelized Ulleung Basin plain with higher accumulation rate (mean 26.1 cm/kyr) than that during the Holocene period. The very thick turbidite mud of LGM period occurs only in the unchannelized Ulleung Basin plain, which is not correlated with other layers in the channelized basin plain.